Question

When does the lac-operon in E.coli become “switched on”
A) Repressor binds to operator
B) RNA polymerase binds to operator
C) Lactose is present and it binds to the repressor
D) Lactose is present and it binds to RNA polymerase

Answer 1

Hint: The lac operon also known as lactose operon of E. coli contains genes involved in lactose metabolism which is used for metabolizing the sugar lactose. It is a single promoter transcribed as a single mRNA.

Complete answer:
-Operon means a segment of DNA.
-For the transport and metabolism of lactose in E.coli lac operon is used.
-Escherichia coli is a type of bacteria that lives in the intestines of humans or also found in the gut of some animals.
-The lac operon in E. coli controls the organic phenomenon of the enzymes that digest lactose within the cell.
-A repressor may be a sort of protein that inactivates the expression of the lac operon genes by binding to the DNA of the lac operon.
-To release the operon lactose molecule bind to the repressor protein due to which its shape also changes. Due to this RNA polymerase binds to the promoter and transcribes the genes of the operon.
-Normally, the lac operon is turned off but a repressor protein binds the operator (control) region upstream of the operon preventing transcription.
-In the presence of lactose the lac operon will activate and therefore the genes are going to be expressed.
-Hence, the lac operon is “switched on” when lactose is present and it binds to RNA polymerase.

Thus the correct answer is option ‘D’.

Note: The lac operon of E.coli is expressed only when lactose is present and glucose is absent. Two regulators turn the operon "on" and "off" in response to lactose and glucose levels: the lac repressor and catabolite activator protein (CAP).